1. Field of the Invention
The present invention relates to a heating cooker employing heating lamps such as halogen lamps as a heat source that emits infrared rays to heat an object.
2. Description of the Prior Art
Various heating cookers employing heating lamps such as halogen lamps that emit infrared rays have been proposed in recent years. A heating cooker of this type has a heat resistant top plate over the halogen lamps. An object to be heated is placed on the top plate. The top plate is made of, for example, tempered glass that transmits the infrared rays. A reflector is arranged under the halogen lamps to reflect the infrared rays toward the object on the top plate. The heating cooker heats the object not only directly with the infrared rays but also with heat of the halogen lamps conducted through the top plate.
The top plate of the heating cooker has, however, a relatively large heat capacity and low thermal conductivity, so that a temperature of the object on the top plate may not be increased rapidly at the start of heating.
To cope with this, the heating cooker usually employs a large output heat source such as halogen lamps having an output capacity of 2 kw (kilowatts), and other techniques for improving thermal efficiency.
For example, a heat source of the heating cooker may comprise a plurality of halogen lamps, and upper and lower heat insulators each having an annular shape, for holding the halogen lamps between them. The top face of the upper heat insulator is in contact with the bottom face of the top plate. The lower heat insulator is arranged in a receiver. The top plate, upper and lower heat insulators, and receiver define a sealed interior space of the heat source.
This arrangement may cause, however, another problem that, when the halogen lamps are continuously operated, an ambient temperature around the halogen lamps inside the heat source sometimes exceeds the heat resistive temperature (about 850.degree. C.) of a quartz glass tube forming a bulb of each halogen lamp.
To deal with this, a thermostat is disposed to detect the temperature inside the heat source. When the temperature exceeds a preset value, the thermostat operates to stop supplying electric power to the halogen lamps, or to lower the heat output of the halogen lamps. The temperature inside the heat source, however, changes slowly and does not quickly follow the operation of the thermostat. This causes the thermostat to operate repeatedly. Such repetitive operation of the thermostat may unnecessarily lower the heat output of the halogen lamps and the temperature inside the heat source, thereby the service lives of the halogen lamps.